Diatoms are microscopic, unicellular aquatic plants which are distinguished by an opaline silica shell. Upon dying, the diatoms sink to the bottom of the water and the shells form a bed which may be several hundreds of feet thick. These beds may eventually elevate above the surrounding water level, and through prolonged and repeated leaching, become purified. Material from these beds is called diatomaceous earth or diatomite.
The remarkable properties of diatomaceous earth make it industrially useful. Its inertness and various shapes make it an excellent filteraid and as such it is used extensively throughout the world.
In filteraid manufacture, diatomaceous earth is mined by the open quarry method, transported to a mill, crushed, screened, and put into storage bins. Material from these bins is dried, extraneous material such as clay and chert is removed, and the diatomaceous earth ore is then calcined in a rotary kiln. After calcining the filteraid is milled, classified, bagged, and shipped to the customer.
Filteraids are generally divided into two types according to the method of calcination. In one method, straight calcining, milled diatomite from the storage bins is passed through a rotary calciner. The resulting products are called "pinks" or calcined diatomaceous earth. In the other method, flux-calcining, the purified crude from the storage bin is mixed with a flux, usually sodium carbonate, and passed through a rotary calciner. The resulting products are called "whites" or flux-calcined diatomaceous earth. Both white and pink filteraids are used extensively in the brewing and pharmaceutical industries.
Diatomaceous earth filteraid materials normally contain iron in various forms, one or more of which may be soluble in the liquid being filtered. In the brewing of beer, for example, the beer is preferably clarified by filtering through diatomaceous earth filteraid material. However, the iron content of the normal filteraids which is soluble in beer has a deleterious effect on the beer.
The presence of iron in beer is widely known. W. P. K. Findlay, Modern Brewing 256 (1971) establishes that the presence of iron in beer in excess of several micrograms per gram can cause a severe decline in stability or shelf life. Similarly, soluble iron affects the stability of various pharmaceuticals and distilled spirits. C. J. King, Separation Processes 18 (1971). Reduction of beer-soluble iron in diatomaceous earth filteraids prolongs the shelf life of the beer.
In view of the value of the diatomaceous earth filteraids in the clarification of beer in the brewing process, there has been a great deal of interest over a long period of time in economical and efficient ways and means for removing the beer-soluble iron from the filteraid material or changing the form of the iron content so as to render it substantially insoluble in beer.
It was discovered, for example, that by some internal chemical process, a certain percent of the beer-soluble iron becomes insoluble to the beer, merely by the aging of the filteraids before use in the brewing process. During aging of the filteraid for approximately one month, up to 20% of the beer-soluble iron may be rendered beer-insoluble.
U.S. Pat. No. 665,652 to Enzinger describes a process for producing a diatomaceous filteraid useful for filtering beer. The diatomite is heated in an aqueous solution of twenty percent hydrochloric acid and one percent nitric acid for some hours in an autoclave at a temperature of about 120.degree. C. The steps of filtering, washing and pressing follow. However, the process fails to reduce beer-soluble iron to the extremely low levels sufficient to meet the more sophisticated demands of today's beer producer.
Without focusing specifically on the reduction of beer-soluble iron content, others have sought to reduce water and acid-soluble iron contents of diatomaceous earth filteraids. U.S. Pat. No. 2,701,240 to Bregar discloses a process to reduce acid-soluble iron content in diatomite filteraids from about 3 percent to less than 0.02 percent, with one example reporting reduction to 0.005 percent. The process includes flux-calcining followed by contacting with acid, heating and agitating from fifteen minutes to several hours, filtering, preferably washing, drying and sometimes milling. Preferably, excess of acid is employed. The examples in the Bregar specification indicate that the amount of acid used is about 600 percent by weight based on dry filteraid weight. The process does not employ calcined diatomaceous earth filteraids.
U.S. Pat. No. 1,992,647 to Schuetz describes a method of producing a diatomaceous earth filteraid with less than 0.03% water-soluble material. The preferred embodiment of the process includes treating comminuted, but not calcined, diatomaceous earth with a strong mineral acid, suitably sulfuric acid, leaching with water, drying, milling, calcining, treating with a dilute mineral acid, leaching, drying and milling.
All of these prior art methods fail to reduce beer-soluble content in diatomaceous earth filteraids to the extremely low levels desired in the brewing industry. Further, the methods involve either processing for a considerable period of time or otherwise complicated and expensive procedure. Most of the methods also employ leaching processes to remove impurities.
Accordingly, a primary object of the present invention is to efficiently and economically reduce beer-soluble iron in diatomaceous earth filteraids to an extremely low level to meet the more sophisticated demands of today's beer producer.
A further object is to provide a process of adding small quantities of a dilute acid solution to the filteraid to tie-up the beer-soluble iron rather than to remove it. The fact that the small residual acid used is allowed to stay with the product further reduces the processing cost.